Soil is life
I’m not sure if you’re aware, as I was not aware of this before coming to Lincoln University, but soil is very much alive. There are millions of organisms in a single handful of soil. These critters perform the majority of the ecosystem services provided by soils.
In New Zealand, these organisms range from burrowing animals, to soil-dwelling invertebrates, to fungi, to microscopic bacteria and much more. So next time you walk on soil, think about how much life is beneath your feet.
For plants, a major component of successful growth and survivability is the soil microbiota, such as bacteria, fungi, viruses, nematodes (tiny worms), and many more. I have much love for fungi in the soil microbiota, especially mycorrhizal fungi that are essential for plant growth.
Mycorrhizal fungi form mutualistic relationships (beneficial for both organisms) with plant roots. The fungi extend their network to increase the nutrient intake in exchange for carbohydrates that the fungi cannot produce itself.
There are two major types of mycorrhizal fungi, ectomycorrhizal fungi, which infect the outside of plant roots, and arbuscular mycorrhizal fungi (AMF or endomycorrhizal), which infect the inside of plant roots.
I did a project on ectomycorrhizal fungi abundance on red beech tree roots for SCIE393 where I took this photo of an ectomycorrhizal fungal fruiting body (mushroom) under the microscope (which I thought was very cool). This is really where my love for mycorrhizal fungi started to blossom.
Exotic plants are strong invaders
That’s enough about me for now, time to talk about invasions from non-native plants. As I’m sure everyone who lives in New Zealand is aware, we live in an invaded country, full of exotic predators, pests, and weeds.
While everyone tends to think of the worst invaders are pests like possums and mustelids, the worst invaders are actually the weeds. Approximately half of all the vascular plants in the wild in New Zealand are non-native. There are around 25,000 exotic plant species that have been introduced to New Zealand, with around 2,700 of these becoming wild. Compared to around 2,500 native plants, this is an insane numbers of exotic plants and so to conserve our native species, we must do something about all of these exotic species.
One of the worst exotic plant species are wilding pines, such as lodgepole pine and radiata pine. Lodgepole pine was introduced into New Zealand in 1880 to combat erosion. Burning of forests, overgrazing, and introduced browsers, such as deer, goats and rabbits, were clearing the hillsides of our native plants, leading to more slips. The New Zealand government decided to plant exotic plants, specifically Douglas-fir and lodgepole pine to cover the hillsides and reduce erosion.
These pines did stop the erosion, however, what was not anticipated was the enormous dispersal ability of these wilding pines. Seeds of wilding pines can survive for a decade in their cones, grow roughly anywhere, and can travel kilometers from their parent. There is now roughly 800,000 hectares of wilding exotics in the South Island alone, two thirds of which is lodgepole pine!
Be careful what you plant!
This paper headed by Joanna Green has contributions from my lovely thesis supervisor Lauren Waller. In it they describe the effects of planting non-native plants on the soil microbiota, and how that can impact what grows there afterwards. They were specifically looking at how well lodgepole pines grow in soils that have been used and conditioned by exotic plant species.
Joanna and Lauren started by using soils that had already been conditioned by 19 native and 20 exotic plant species from another two experiments by Warwick Allen and Lauren Waller. The soil microbiota, the nutrients in the soil, and other factors, like the pH of the soil, had been changed to best suit the plant that had been grown in that soil.
An example of part of the soil microbiota that was conditioned were the ectomycorrhizal fungi. These fungi are required for successful growth by some of our native species, such as our beech, mānuka, and kānuka species. However, exotic species use different species of ectomycorrhizal fungi than our native species. Therefore, when exotic species are grown in soil they will grow well with their species of ectomycorrhiza.
Basically, think of it like this. If you owned a garden and really liked roses, you would grow a lot of roses. You might grow some other plants but the focus of the garden would be on roses. Now, if someone else came in and took over the garden for themselves, they might not like roses as much but might really like petunias. They will then plant petunias in place of where the roses used to be, altering the state of that garden.
In terms of ectomycorrhiza, they require associations to survive, so the native ectomycorrhiza species that were in the soil will die after their native plant partners are gone. Exotic ectomycorrhiza species associated with exotic plant species will then survive. Due to this, it will become a lot more difficult for our native species to grow back in that soil due to a lack of ectomycorrhiza that they can grow with. Conversely, it will be a lot easier for exotic plant species to grow in partnership with a large number of exotic ectomycorrhiza.
Anyway, back to the paper. The researchers found that lodgepole pine grew better in soils that had been conditioned by exotic species, such as black alder, cocks-foot, spear thistle, gorse, and many more. And this was regardless of whether pines had been planted there or not! Lodgepole pine will have a much easier time establishing and invading into areas that have previously had exotic species growing there.
So why does this matter?
To conserve our native plants, sometimes we must plant them in areas that have previously grown exotic species. Joanna and Lauren’s research shows that the soil will be conditioned to favour growth of exotic species, and so growing native plants will be more difficult. It will also be a lot easier for exotic species, such as lodgepole pine, to re-invade and take over the conservation site. Therefore, conservation managers must be aware of this soil legacy and decisions must be made in order to manage this re-invasion potential.
So, next time you want to plant an exotic plant, think about the soil, and whether you would rather plant a native instead.
This article was prepared by postgraduate student Lucas Watkin, Master of Science in Conservation and Ecology, for an assignment in ECOL608 Research Methods in Ecology.
Green, J. L., Waller, L. P., Allen, W. J., Orwin, K. H., Pelser, P. B., Smaill, S., & Dickie, I. A. (2025). Plant-soil feedback from non-native communities increases pine invasion and re-invasion potential. Plant Soil, 514(2), 2461-2474. https://doi.org/10.1007/s11104-025-07528-x
Featured image: Did somebody order pines? The rapid invasion of black pine into well grazed high country pasture – Photo by Jon Sullivan (CC BY-NC 2.0)